T

Figure 2.12 The impact of complete positive genotypic assortative mating (like genotypes mate) or self-fertilization on genotype frequencies. The initial genotype frequencies are represented by D, H, and R. When either of the homozygotes mates with an individual with the same genotype, all progeny bear their parent's homozygous genotype. When two heterozygote individuals mate, the expected genotype frequencies among the progeny are one half heterozygous genotypes and one quarter of each homozygous genotype. Every generation the frequency of the heterozygotes declines by one-half while one-quarter of the heterozygote frequency is added to the frequencies of each homozygote (diagonal arrows). Eventually, the population will lose all heterozygosity although allele frequencies will remain constant. Therefore, assortative mating or self-fertilization changes the packing of alleles in genotypes but not the allele frequencies themselves.

progeny of the heterozygous genotypes. If the process of complete assortative mating continues, the population rapidly loses heterozygosity and approaches a state where the frequency of heterozygotes is 0.

As an example, imagine a population where p = q = 0.5 that has Hardy-Weinberg genotype frequencies D = 0.25, H = 0.5, and R = 0.25. Under complete positive assortative mating, what would be the frequency of heterozygotes after five generations? Using Fig. 2.12, at time t = 5 heterozygosity would be H(1/2)5 = H(1/32) = 1/64 or 0.016. This is a drastic reduction in only five generations.

Genotype frequencies change quite rapidly under complete assortative mating, but what about allele frequencies? Let's employ the same example population with p = q = 0.5 and Hardy-Weinberg genotype frequencies D = 0.25, H = 0.5, and R = 0.25 to answer the question. For both of the homozygous genotypes, the initial frequencies would be D = R = (0.5)2 = 0.25. In Fig. 2.12, the contribution of each